1. Field of the Invention
The present invention relates to an improved process for treating sludge produced by water treatment processes, to recover coagulant used to effect flocculation of solids in the untreated raw water.
Approximately 70% of the water treatment plants in the United States are turbidity removal plants. Water treatment plants utilize coagulants such as aluminum sulfate (alum) and ferric sulfate to remove the turbidity in the raw water. The coagulant added to the raw water reacts with the water's natural or added alkalinity to produce an insoluble base form of the coagulant as a precipitate. This coagulant precipitate removes suspended materials such as silt, dissolved or colloidal organic material, microscopic organisms, and colloidal metallic hydroxides present in the raw water.
After the coagulant precipitate and other suspended solid material (collectively called coagulant-containing sludge) settle, the treated water is separated from the coagulant-containing sludge. Coagulant-containing sludge is generally dilute; typically, the suspended solids content for alum sludges ranges from 1 to 15%. Landfill disposal of alum sludge requires at least 10% to 20% solids content. Currently, the most effective means of sufficiently dewatering alum sludge for disposal is mechanical filter pressing. Although mechanical filter pressing yields dewatered alum sludge possessing at least 30% to 50% solids, the initial equipment cost and subsequent operational costs are high.
The cost of alum sludge disposal is related to its solids content and dewatering characteristics. It is well known that recovery of the alum from the alum sludge improves the dewatering characteristics of the remaining solids and reduces the overall quantity of solids which require disposal. Thus, recovery of alum from the alum sludge reduces the overall disposal costs.
The primary objective of the typical alum recovery process is to acid-solubilize the maximum recoverable amount of alum present in the alum sludge. In an effort to obtain maximum alum recovery, acid is added to achieve a predetermined pH.
One major drawback common to known alum recovery processes involves gradual buildup of impurities in the recycled alum. The addition of acid to the alum sludge converts the base form of the alum to the acid soluble form and simultaneously solubilizes numerous impurities as well, e.g., iron, manganese, and "color." These acid soluble impurities comprise metallic and organic materials as well as impurities commonly present in commercial acid. Repeated alum recovery and recycling operations cause a steady accumulation of acid soluble impurities in the recovered alum. Gradually, the quality of the treated water declines due to contamination with these same acid soluble impurities. Eventually, this situation forces the discarding of all of the recovered alum and total replacement with commercially available coagulant. Fifteen water treatment facilities built in Japan between 1965 and 1972, including five in the Tokyo area, previously used such an alum recovery process. Concern over contamination of treated water by the concentration of acid soluble impurities in the recovered alum evident at these facilities, however, prompted officials to later abandon use of the recovery process at Japanese facilities built after 1972. See, Committee Report, Water Treatment Plant Sludges-An Update of the State of the Art: Part I, 70 J. A.W.W.A. 498 (1978).
The second drawback associated with known alum recovery processes is linked to the use of mechanical filter presses to recover the alum from the alum sludge after acid has been added and to dewater the remaining solids. Mechanical filter presses are both expensive to buy and operate. Moreover, removal of the dewatered solids is a slow process requiring extended cessation of the alum recovery process.
2. Description of the Prior Art
U.S. Pat. No. 3,959,133 to Fulton discloses a process which utilizes a mechanical filter press in a vertical configuration to recover the alum from the alum sludge and dewater the remaining solids. To overcome the inherent problem of acid soluble impurity accumulation in the recovered coagulant, Fulton's process requires a complete discarding periodically of the recovered alum and subsequent total replacement with commercially available alum.
Numerous articles have been written which generally describe the principles of alum recovery similar to those discussed in the Fulton patent. One such reference, which describes the Fulton process in detail, is S. L. Bishop, Alternate Processes for Treatment of Water Plant Wastes, 70 Journal of the American Water Works Association (J. A.W.W.A.) 503 (1978). Other references discussing principles of alum recovery include: K. Tomono, The Art of Water Treatment in Japan, 69 J. A.W.W.A. 166 (1977); Committee Report, Disposal of Water Treatment Plant Wastes, 64 J. A.W.W.A. 814 (1972); Committee Report, Water Treatment Plant Sludges-An Update of the State of the Art: Part 1, 70 J. A.W.W.A. 498 (1978); J. T. O'Connor and J. T. Novak, Management of Water Treatment Plant Residues, Proceedings A.W.W.A. Seminar on Water Treatment Waste Disposal (1978); W. R. Inhoffer, Filter Washwater and Alum Sludge Disposal a Case Study, Proceedings A.W.W.A. Seminar on Water Treatment Waste Disposal (1978); G. P. Westerhoff, Minimization of Water Treatment Plant Sludges, Proceedings A.W.W.A. Seminar on Water Treatment Waste Disposal (1978); G. P. Fulton, Recover Alum to Reduce Waste-Disposal Costs, Processing Water Treatment Plant Sludge A.W.W.A. (1974); A. P. Black, B. S. Shuey, and P. J. Fleming, From Lime-Soda Softening Sludges Recovery of Calcium and Magnesium Values, 63 J. A.W.W.A. 616 (1971); and D. A. Cornwell and J. A. Susan, Characteristics of Acid Treatment Alum Sludges, 71 J. A.W.W.A. 604 (1979).
Recently, researchers have investigated recovering alum from water treatment plant sludge by liquid-ion exchange. This method uses organic solvents to recover highly pure concentrated alum from sludge by liquid-ion exchange. The liquid-ion exchange is a process step added after acid is added to the alum sludge and is used to eliminate acid soluble impurities from the recovered alum. See G. P. Westerhoff and D. A. Cornwell, A New Approach to Alum Recovery, 70 J. A.W.W.A. 709 (1978) and D. A. Cornwell and R. M. Lemuryon, Feasibility Studies on Liquid-Ion Exchange for Alum Recovery From Water Treatment Plant Sludges, 72 J. A.W.W.A. 64 (1980). The economics of liquid-ion exchange have yet to be proven.
Thus the prior art in no way suggests an improved, economical coagulant recovery process characterized by freedom from unwanted acid soluble impurity accumulation and utilizing an inexpensive non-mechanical substantially horizontal filtering medium to produce a recovered coagulant filtrate with enhanced coagulative properties.